A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
A lithographic apparatus includes a substrate table onto which the substrate is to be accurately loaded before starting the actual transfer of a pattern on the substrate. The substrate may be loaded onto the table by first placing it on a set of three slender ejector pins upwardly projecting out of holes which are provided in the table. These pins are constructed so as to be moveable in the vertical Z-direction, that is to say in a direction perpendicular to the upper surface of the substrate and/or the table, commonly indicated as the X, Y plane. The pins are connected to one another and their up- and downward moving takes place using a common drive. After the substrate is placed on the pins, they are lowered until the substrate is attracted by vacuum device (e.g. a vacuum pump) integrated in the table. After that, the pins are further lowered until the substrate is fully supported by the table and clamped thereon. The substrate is then no longer supported by the pins.
Offsets and tilts in the loading of the substrates may have a negative impact on the overlay during the transfer of the pattern. It has also been found that mere is some significant spread in how the substrate is loaded to the table between different chucks for holding the substrate table.
In order to be able to improve the loading process of the substrate, and in particular to compensate for the effects of imperfections in the construction of the ejector pins relative to the table, a calibration process may be performed. From this calibration process, an optimum take over height for the substrate to be taken over from the ejector pins by the substrate table may be determined and vice versa. Also, a possible tilt of the pins with respect to the table may be corrected via a tilt calibration.
The height calibration is currently done as follows: the vacuum of the substrate table is switched on, and the pins with the substrate supported thereon are lowered stepwise. When the substrate has come at a certain distance from the table, it is pulled by the vacuum towards the table. This causes a change in the vacuum forces, which change is detected. The moment of change of the vacuum forces is then reduced to the height of the ejector pins at that moment. This height can be subtracted with a predefined offset from a calibration starting height in order to obtain the required optimum take over height.
This present calibration method for determining the optimum take over height has a limited accuracy, in particular to about 0.2-0.3 mm.
The present tilt calibration is currently done with the aid of a mechanical tool. The accuracy of this mechanical tilt correction is also limited, and in practice has appeared to be about 300 μrad.